My studies on the organization of the motor neuron pools for the leg and wing muscles of the chick are close to completion but some details remain to be worked out. These experiments and the experiments on the motor pools which innervate specific muscles of grafted supernumerary limbs will be completed and written up for publication in the next year. The discovery of an apparent exception to the rule of motor neuron specificity for muscles derived from either the ventral or dorsal muscle mass in a limb which had been grafted in 180 degrees reversed anterior-posterior orientation will be investigated further. Several other muscles have been mapped in A-P reversed legs and their innervation follows the muscle mass specificity rule. The reason for this single exception remains unknown. In nearly all of the experimental cases, the frafted leg or wing and the original host's leg have been processed histologically so that the peripheral innervation patterns can be reconstructed. Preliminary data show that the points of entrance and distribution of the nerves supplying the two lims are normal, even with the reduced number of innervating segments. The material will also be examined to determine whether any muscles have atrophied or have failed to develop. My laboratory has been making electromyographic recordings from many muscles of the hatched chick as it walks. Once we know the normal pattern of motor output for the various motor pools of the spinal cord, we will be in a position to determine whether the patterns of motility of a grafted limb are normal or whether they reflect only the motor patterns of the motor pools of the restricted number of spinal segments which innervate the grafted limb. Recordings will probably have to be made from chicks in ovo since 5-limbed animals rarely reach hatching. Preliminary experiments have been performed attempting to substitute wing spinal segments for some of the lumbar segments. No embryos have yet survived long enough to attempt HRP injections to label the motor pools. These experiments will be continued.